Method for Producing a Hot-Formed and Hardened Steel Component Coated with a Metallic Anti-Corrosion Coating from a Sheet Steel Product

ABSTRACT

A method for producing a steel component with a metallic anti-corrosion coating from a sheet steel product comprising at least 0.4% by weight Mn is disclosed. The sheet steel product is annealed in a continuous furnace under an annealing atmosphere containing up to 25% by volume H 2 , 0.1% to 10% by volume NH 3 , H 2 O, N 2 , and process-related impurities as the remainder, at a dew point between −50° C. and −5° C. at a temperature of 400 to 1100° C. for 5 to 600 s. The annealed sheet steel product has a 5 to 200 μm thick nitration layer with a particle size finer than the particle size of the inner core layer. Once coated with a metallic protective layer, a blank is separated from the annealed sheet steel product, heated to an austenitising temperature of 780 to 950° C., hot-formed, and cooled so that a hardened structure forms.

The invention relates to a method for producing a hot-formed and hardened steel component coated with a metallic anti-corrosion coating from a sheet steel product, which has an Mn-content of at least 0.4% by weight.

As reported in the Article “Potenziale für den Karosserieleichtbau” (Potential for lightweight body construction), published in the Exhibition Newspaper of ThyssenKrupp Automotiv AG for the 61^(st) International Car Exhibition in Frankfurt, from the 15^(th) to 25^(th) Sep. 2005, hot-forming hardening is applied in practice in particular to the production of high-strength body components from boron-alloyed steels. A typical example of a steel of the type in question here is the steel known by the designation 22MnB5, which can be found in the Key to Steel 2004 under the material number 1.5528.

A steel that is comparable with the steel 22MnB5 is known from JP 2006104526A. This known steel contains, apart from Fe and inevitable impurities (in % by weight), 0.05 to 0.55% C, max. 2% Si, 0.1 to 3% Mn, max. 0.1% P and max. 0.03% S. To improve the hardenability, contents of 0.0002 to 0.005% B and 0.001 to 0.1% Ti can be added to the steel. The respective Ti-content is used here to set the nitrogen present in the steel. The boron contained in the steel can thus develop its strength-increasing effect as completely as possible.

According to JP 2006104526 A, metal sheets, which are then preheated to a temperature above the Ac₃ temperature, typically ranging from 850 to 950° C., are firstly manufactured from the steel composed in this manner. During the subsequent rapid cooling starting from this temperature range and taking place in the pressing tool, the martensitic structure ensuring the high strengths aimed for is formed in the component press-formed from the respective sheet metal blank. It is advantageous here that the sheet metal parts heated to the temperature level mentioned can be formed into components that are formed in a complex manner at relatively low forming forces. This also applies, in particular, to sheet metal parts of the type which are manufactured from high-strength steel and provided with an anti-corrosion coating.

The hot forming of zinc-plated sheet steel products into high-strength or very high-strength steel components presents a particular difficulty. If a steel sheet provided with a metallic anti-corrosion coating has to be heated for the hot-forming and a possible subsequent hardening or a hardening carried out in combination with the hot-forming, to a temperature, which is above the melting temperature of the metal of the protective coating, there is a risk of so-called “liquid metal embrittlement”. This embrittlement of the steel occurs when molten liquid metal of the coating penetrates into the notches being formed on the surface of the respective sheet steel product during forming. The liquid metal reaching the steel substrate settles there at the particle boundaries and thus reduces the maximum absorbable tensile and compressive stresses.

The risk of liquid metal embrittlement in sheet steel products produced from higher-strength and high-strength Mn-containing steels proves to be particularly critical. These steels only have a limited ductility and as a result tend to form cracks close to the surface and close to the particle boundary as a result during their forming.

It is generally known from DE-OS 18 13 808 that the corrosion and oxidation resistance of a steel sheet can be improved by a nitriding treatment, by means of which an edge layer that is 2.5 to 19 μm in thickness and close to the surface is produced with a nitrogen content that is elevated relative to the core region of the steel sheet. The nitration layer has good adhesion.

It is furthermore known from DE 691 07 931 T2 that in a region close to the surface of sheet steel products consisting of low-carbon steels and intended for the construction of motor vehicle bodies, higher C- or N-contents can be produced by a carbonising or nitriding treatment in order to improve the processability of the relevant sheet steel products.

These measures in the prior art are not in connection with higher-strength or high-strength steels, which have Mn-contents of at least 0.4% by weight, typical Mn-contents of the steels processed according to the invention being in the range from 0.4 to 0.6% by weight, in particular 0.6 to 3.0% by weight.

The C-content of the sheet steel products processed according to the invention is typically more than 0.06% by weight and less than 0.8% by weight, in particular less than 0.45% by weight.

Examples of the steels processed according to the invention, to adjust their respective properties, may contain up to 0.2% by weight Ti, up to 0.005% by weight B, up to 0.5% by weight Cr, up to 0.1% by weight V or up to 0.03% by weight Nb.

The nitriding, or the inner nitration, assume the presence of nitrogen capable of diffusion. This prerequisite is satisfied when the nitrogen is present in statu nascendi.

The nitration generally takes place by annealing the respective sheet steel products in an ammonia-containing H₂-N₂ annealing gas atmosphere. Ammonia and nitrogen are available there as nitrogen dispensers. Ammonia gas splits into nitrogen and hydrogen at atmospheric pressure and temperatures above 400° C. while doubling its volume. The dissociation of ammonia gas can be described by the following reaction equation:

2NH₃->2[N]+3H₂

Against the background of the prior art described above, the object of the invention was to disclose a method, which, while minimising the risk of the development of metal-induced cracks, economically allows a high-strength steel component to be produced.

This object was achieved according to the invention in that when producing a high-strength steel component, the working steps disclosed in claim 1 are carried out.

Advantageous configurations of the invention are disclosed in the claims that depend on the respective independent claims and will be described in detail below as will the general inventive idea.

The method according to the invention for producing a steel component coated with a metallic anti-corrosion coating is based on the idea of carrying out a nitriding treatment on the sheet steel product before it is hot-formed, a finely structured edge layer being produced in the sheet steel product by means of said nitriding treatment. On the one hand, this edge layer improves the forming properties of the surface-finished steel product for the hot-forming.

On the other hand, the edge region of the sheet steel product nitrided in the manner according to the invention proves to be surprisingly helpful in avoiding metal embrittlement of the fine steel sheet during the hot-forming. The nitration zone thus brings about a significant increase in the particle boundary surfaces/phase boundary surfaces during the hot-forming process, which counteracts the crack failure of the material as a consequence of metal material of the coating penetrating into the structure of the steel substrate. Moreover, an unusually high iron diffusion is adjusted in the coating. As a result, the coating becomes thermally more stable, in particular when processing coatings based on zinc.

In order to utilise the positive influences of the edge layer nitriding carried out according to the invention that is summarised above, the method according to the invention comprises the following working steps:

-   -   A sheet steel product made of a steel having an Mn-content of at         least 0.4% by weight is provided. If a sheet steel product is         mentioned here this then means, in general, steel sheets, bands,         blanks or the like. A sheet steel product of this type may be         processed in the hot-rolled or cold-rolled state in the manner         according to the invention. It is also conceivable to combine         different steel blanks to form a sheet steel product then         processed in a manner according to the invention, one of the         steel blanks consisting of a steel of the type disclosed in         claim 1.     -   The sheet steel product is annealed in a continuous furnace         under an annealing atmosphere, which contains up to 25% by         volume H₂, 0.1 to 10% by volume NH₃, H₂O and N₂ as the remainder         as well as process-related inevitable impurities and which has a         dew point of between −50° C. and −5° C. The holding temperature,         at which the sheet steel product is held for a holding period of         5 to 600 s, in this case is 400 to 1100° C. As a result, owing         to this nitriding-annealing treatment, a 5 to 200 μm thick         ductile nitration layer adjoining its free surface is present on         the sheet steel product, the particle size of which nitration         layer is finer than the particle size of the inner core layer         covered by the edge layer and formed by the basic material of         the sheet steel product.     -   After the production of the nitration layer, the sheet steel         product annealed in the manner disclosed above is coated with a         metallic protective layer. The invention utilises the         recognition here that the risk of a liquid metal embrittlement         can be minimised in that by a targeted modification of the         region of the sheet steel product close to the surface, the         temperature range susceptible to liquid metal embrittlement can         be displaced in such a way that it does not coincide with the         temperature interval typical for the hot-forming.     -   Blanks are separated from the sheet steel product coated with         the metallic protective layer.     -   If the forming takes place in two or more stages, the blank may         optionally be preformed at this point. The preforming can go so         far here that after the preforming, the shape of the blank         virtually completely corresponds to the shape of the finished         component. Typically, the preforming takes place with a cold or         semi-hot blank heated below the austenitising temperature. With         a one-stage forming carried out only by hot-forming, the         preforming is dispensed with.     -   For the hot-forming, the blank is heated to an austenitising         temperature of 780 to 950° C.     -   The hot-forming of the heated blank into the finished steel         component then takes place.     -   The steel component obtained is then subjected to a cooling, in         which, starting from the austenitising temperature, accelerated         cooling takes place. The cooling of the steel component takes         place here in such a way that a hardened structure forms in the         sheet steel product.

The hot-forming and the hardening may take place “in one stage”. In this case, the hot-forming and the hardening are carried out in one step together in a tool. On the other hand, in the two-stage process, the working steps “forming” and “producing the heat treatment or hardened structure” are carried out separately from one another.

Surprisingly, when applying the annealing conditions predetermined according to the invention, it is possible to achieve the desired nitriding depth even with very short conditioning times. Thus, the method according to the invention is distinguished, in particular, in that it can be carried out in a particularly economical manner using a continuous furnace. This makes it possible to incorporate the method according to the invention in continuous production processes, which assume high belt speeds, such as is the case, for example, in hot-dip galvanising plants, in which steel bands are heat treated and are hot-dip coated with the anti-corrosion coating in a continuous run.

Iron surfaces present in the reaction chamber catalytically promote the dissociation. A part of the nitrogen atoms released at the moment of disintegration may diffuse into the iron material.

Nitrogen transfer takes place in a plurality of part steps:

-   -   Transportation to the workpiece surface     -   Adsorption on the surface     -   Penetration of the surface (absorption)     -   Diffusion into the workpiece interior

Because of the increased nitrogen solubility in the austenite, it is expedient to carry out the annealing intercritically, i.e. in the two-phase area α/γ-Fe. Independently of whether the subsequent coating is carried out with the metallic protective layer continuously or piece-wise, the result of the nitriding treatment can accordingly be optimised under the conditions generally provided in practice in a particularly economical and environmentally compatible manner in that at least one of the following conditions is adhered to:

-   -   the H₂-content of the annealing atmosphere is at most 10% by         volume,     -   the NH₃-content of the annealing atmosphere is at most 5% by         volume,     -   the dew point of the annealing atmosphere is −40° C. to −15° C.,     -   the holding temperature of the annealing is 680 to 840° C.,     -   the holding period of the annealing is 30 to 120 s.

It is decisive for the success of the invention that during the annealing treatment according to the invention, a nitration edge layer is adjusted, the particle size of which is significantly finer than the particle size of the core layer of the sheet steel product that is not nitrided during the annealing. Practical tests have shown that according to DIN EN ISO 643, the characteristic particle size of the nitration layer is smaller by at least 2 than the characteristic particle size of the basic material (core layer) of the annealed sheet steel product before the heating and hot-forming of the blank.

During the method according to the invention, a nitrided edge layer is produced in a targeted manner. The thickness of this finely structured, optionally only partly recrystallized nitration layer is determined by the nitration hardness depth determined according to DIN 50190-3. Accordingly the nitration hardness depth is the spacing from the surface to the point of the steel substrate, at which the hardness corresponds to the core hardness+50 HV. In this manner, a hardness is adjusted in the nitrided edge layer region of the sheet steel product close to the surface, which is at least 25% higher than the hardness of the core region, i.e. Hv(nitrided)/Hv(core region)≧1.25.

Typically, in a sheet steel product processed according to the invention, the thickness of the nitrided edge region after the annealing treatment is >5 μm and <200 μm.

A configuration of the invention that is particularly advantageous in practice is characterised in that the coating of the sheet steel product with the metallic protective layer takes place by means of a hot-dip coating, which is completed in a work sequence carried out continuously following the annealing treatment. In this case, the annealing treatment carried out according to the invention is carried out at the same time as the surface conditioning for the downstream surface finishing by means of a heterogeneous annealing gas-metal reaction.

It is particularly advantageous here to use the method according to the invention in a hot-dip coating line, as the annealing treatment in this case may comprise the edge nitriding, surface conditioning and recrystallisation of the basic material and the hot-dip galvanising can then be carried out in a continuous method sequence in-line following the annealing treatment. In this case, it is basically conceivable to flood the furnace section through which the sheet steel product runs with NH₃-containing gas over its entire length. In order to not subject all the components of the continuous furnace to the nitriding atmosphere, it may also, however, be advantageous to separate a portion of the furnace section from the other portions of the furnace and to only load this separated portion with the NH₃-containing atmosphere.

In order, in the case of a hot dip coating of the annealed sheet steel product carried out, in particular, as a hot-dip galvanising, to ensure optimum adhesion of the coating on the steel substrate, before the hot-dip coating, an oxidation of the surface of the sheet steel product can be carried out.

In the course of the surface finishing of a sheet steel product produced according to the invention, preferably carried out by hot-dip coating, coating systems known per se can be applied to the steel substrate, which are based on Zn, Al, Zn—Al, Zn—Mg, Zn—Ni, Zn—Fe, Al—Mg, Al—Si, Zn—Al—Mg or Zn—Al—Mg—Si. Following the hot-dip coating, further heat treatment steps can be carried out in order to configure the metallic protective coating in a specific way. If necessary, a diffusion annealing, for example a galvannealing treatment, may also take place continuously after the hot-dip coating.

Alternatively or in addition to the hot-dip finishing taking place in-line, a sheet steel product, on which a finely structured nitration layer has been formed in a continuous annealing in the manner according to the invention, may receive a metallic, a metallic-inorganic or a metallic-organic coating, in that it is coated electrolytically, for example with a Zn, ZnNi or a ZnFe coating, by means of physical vapour or chemical vapour deposition or by means of another metal-organic or metal-inorganic coating method.

In order to further optimise the mechanical properties, an ageing treatment carried out in a conventional manner may follow the annealing treatment according to the invention.

Components that have been hot-formed from a sheet steel product treated according to the invention and then hardened have tensile strengths of 800 to 2000 MPa, in particular 900 to 2000 MPa.

The nitration layer produced according to the invention allows the sheet steel product according to the invention to be heated without problems to an austenitising temperature, in which the sheet steel product has a substantially completely austenitic structure. Even at a temperature as high as this, the risk of embrittlement is minimised in a sheet steel product produced according to the invention even when the sheet steel product is provided with a metallic coating, the melt temperature of which is less than or equal to the heating temperature. The fineness of the particles of the edge layer produced by the nitriding according to the invention prevents a crack formation and thus ensures that no metal of the coating can penetrate into the core region or basic material of the steel substrate.

Owing to the production according to the invention of a finely structured, nitrided nitration layer, in the heat forming process preferably carried out directly, i.e. without prior preforming of the blank, solid metal embrittlement occurring from a metallic coating, in particular a zinc coating, otherwise resulting from diffusion of the coating metal onto the particle boundaries, is therefore prevented. Likewise, the procedure according to the invention, as a result of the coating configuration being produced from the nitriding and advantageous with regard to the Fe/coating metal ratio, prevents the occurrence of solder cracks and thus counteracts the liquid metal embrittlement.

The invention will be described in more detail below with the aid of embodiments, in which:

FIG. 1 shows a vertical microsection of a nitrided-annealed steel sample according to the invention;

FIG. 2 shows a vertical microsection of a non-annealed, rolled comparative sample;

FIG. 3 shows GDOES depth profiles of the nitrogen content of the samples shown in FIGS. 1 and 2;

FIG. 4 shows a vertical microsection of the tensile area of a steel component formed from the steel sample according to FIG. 1;

FIG. 5 shows a vertical microsection of the tensile area of a steel component formed from the rolled steel sample according to FIG. 2.

To check the effects achieved by the method according to the invention, respective rolled cold band samples of a multi-phase steel “MP” and of a steel “WU” conventionally used for hot-forming have been produced. The compositions of the steels MP and WU are given in Table 1.

Two samples manufactured from the steels MP and WU have been subjected to an annealing treatment according to the invention in a continuous furnace for an edge layer nitriding. The annealing parameters applied here are given in Table 2.

For comparison, two further samples manufactured from the steels MP and WU have been subjected in the continuous furnace to a conventional annealing, such as is generally carried out to prepare a hot-dip zinc-plating.

FIG. 1 shows the microsection of the sample treated by annealing according to the invention and produced from the steel WU. It can clearly be seen that a finely structured structural region (nitration layer “N”) close to the surface has been adjusted as a consequence of the procedure according to the invention.

The microsection of the rolled sample also produced from the steel WU, on the other hand, shows no such nitration layer (FIG. 2).

GDOES measurements of the nitrogen content have additionally been carried out on the samples which were rolled or treated by annealing according to the invention and consisted of the steel WU. The GDOES measuring method (“GDOES”=Glow Discharge Optical Emission Spectrometer) is a standard method to rapidly detect a concentration profile of coatings. For example, it is described in the VDI-dictionary “Werkstofftechnik” (Materials Technology), published by Hubert Grãfen, VDI-Verlag GmbH, Dusseldorf 1993.

The result of the GDOES measurements is summarised in FIG. 3, the dashed line showing the nitrogen distribution of the rolled sampled and the solid line showing the nitrogen distribution of the sample treated according to the invention.

FIG. 3 also clearly shows that the sample treated according to the invention has a pronounced nitrided nitration layer N, the thickness of which is about 20 μm.

It was possible to show with the aid of micro hardness measurements that the nitration region N nitrided in the sample that was heat treated according to the invention and produced from the steel WU has a micro hardness of 340 HV and the non-nitrided core region (basic material) K has a hardness of 180 HV. The ratio Hv_(N)/Hv_(K) of the hardness Hv_(N) of the nitrided nitration layer N to the hardness Hv_(K) of the core region K was therefore about 1.9 and therefore significantly above the value of 1.25 predetermined according to the invention for this ratio.

Following the annealment, a surface finishing of the samples took place, in which zinc was electrolytically deposited with a layer thickness of 10 μm on the samples.

Subsequently, the samples consisting of the steel WU were formed and press-hardened by means of the so-called one-stage or direct hot-forming method to form a steel component. For this purpose, the samples were heated over an austenitising period of 6 minutes at an austenitising temperature of 880° C. and then hot-formed in a hot-press forming tool to form a component for a car body.

After the hot-forming, the components obtained were cooled in a manner known per se so rapidly that a hardened structure formed.

A comparison of FIGS. 4 and 5 makes it clear that no crack formation of any kind in the region of the tensile area occurred in the component produced in the manner according to the invention, while clear intercrystalline crack formation is to be noted in the component produced in the conventional manner.

For the zinc-plated and formed samples treated by annealing and produced from the steel MP, comparable results could be shown for the samples treated by annealing according to the invention and conventionally.

The method according to the invention therefore improves the forming properties of surface-finished sheet steel products for hot-forming. For this purpose, by means of a targeted gas-metal reaction during the annealing process before the surface finishing, in a continuous process or piece-wise, an edge nitriding is produced, as a result of which a finely structured nitrogen-containing nitration layer N is adjusted. This nitrided edge layer N, on the one hand, increases the Fe diffusion in the coating and prevents the transportation of the “coating metal” embrittlement producer, i.e. in particular zinc, onto the particle boundaries during the annealing process carried out before the hot-forming.

As a result, components are thus obtained, in which the steel substrate is substantially completely crack-free.

TABLE 1 Remainder iron and inevitable impurities C Mn P Si V Al Cr Ti B Nb Steel [% by weight] MP 0.22 1.7 0.02 0.1 0.002 1.7 0.06 0.1 0.005 0.001 WU 0.22 1.22 0.017 0.25 0.005 0.025 0.13 0.03 0.005 0.003

TABLE 2 Working step According to the invention Annealing treatment Heating rate 10 K/s Holding temperature 800° C. Holding period 60 s Annealing atmosphere 4% NH₃ 96% N₂ Dew point −30° C. Cooling rate to room temperature 20 K/s 

1. A method for producing a steel component that is coated with a metallic anti-corrosion coating from a sheet steel product having an Mn-content of at least 0.4% by weight, comprising the following working steps: providing the sheet steel product; annealing the sheet steel product in a continuous furnace, under an annealing atmosphere containing up to 25% by volume H₂, 0.1 to 10% by volume NH₃, H₂O and N₂ as the remainder as well as process-related inevitable impurities and having a dew point ranging between −50° C. and −5° C., at a holding temperature of 400 to 1100° C., for a holding period of 5 to 600 s, so that the sheet steel product obtained after the annealing treatment has a 5 to 200 μm thick nitration layer, which adjoins its free surface and the particle size of which is finer than the particle size of the inner core layer of the sheet steel product covered by the edge layer; coating the annealed sheet steel product with a metallic protective layer; separating a blank from the sheet steel product; optionally preforming the blank; heating the blank to an austenitising temperature of 780 to 950° C., hot-forming the heated blank to form the steel component, accelerated cooling of the steel component in such a way that a hardened structure forms in the sheet steel product.
 2. The method according to claim 1, characterised in that the H₂-content of the annealing atmosphere is at most 10% by volume.
 3. The method according to claim 1, wherein the NH₃-content of the annealing atmosphere is at most 5% by volume.
 4. The method according to claim 1, wherein the dew point of the annealing atmosphere is −40° C. to −15° C.
 5. The method according to claim 1, wherein the holding temperature of the annealing is 680 to 840° C.
 6. The method according to claim 1, wherein the holding period of the annealing is 30 to 120 s.
 7. The method according to claim 1, wherein the characteristic particle size of the nitration layer of the annealed sheet steel product, determined in accordance with DIN EN ISO 643 before the blank is heated and hot-formed, is smaller by at least 2 than the characteristic particle size of the basic material.
 8. The method according to claim 1, wherein the coating of the sheet steel product with the metallic protective layer takes place by means of hot-dip coating, which is completed in a work sequence carried out continuously following the annealing treatment.
 9. The method according to claim 8, wherein an oxidation of the surface of the sheet steel product is carried out before the hot-dip coating.
 10. The method according to claim 8, wherein the sheet steel product is continuously diffusion-annealed after the hot-dip coating.
 11. The method according to claim 1, wherein the coating of the sheet steel product with the metallic, metallic-organic or metallic-inorganic protective layer takes place by electrolytic coating or a physical vapour or chemical vapour deposition.
 12. The method according to claim 1, wherein the metallic protective layer is a Zn, an Al, a Zn—Al, a Zn—Mg, a Zn—Ni, an Al—Mg, an Al—Si, a Zn—Al—Mg or a Zn—Al—Mg—Si coating.
 13. The method according to claim 1, wherein the austenitising temperature adjusted during the heating is 860 to 950° C.
 14. The method according to claim 1, wherein the hot-forming and the cooling of the component obtained by the hot-forming are carried out in one step.
 15. The method according to claim 1, wherein the component obtained is subjected to a blasting treatment. 